2.3. Heart-rate recording

It is technically relatively easy to record heart-rate over prolonged periods of time. If only a record of heart-rate is needed, this is not an expensive technique and instruments are readily available. If a more elaborate break-down of the data is desired - such as the duration of several different ranges of heart-rate, to indicate differing levels of activity - say 80-100, 100-120, 120-140, over 140 beats/min - then this will become expensive and somewhat complex.

Heart-rate recorders can be purchased to provide almost any information on heart-rate that is desired. However, there is some confusion about what exactly can be derived from heart-rates, and some basic physiological facts need to be repeated.

Firstly, the heart-rate is related to cardiac output. This relation is closer at higher levels of exercise than at moderate and low levels, because of the many other influences on heart-rate, such as posture, effect of meals, psychological factors, the muscle masses involved, and so on. At levels which are most common for infants and children in ordinary daily life, heart-rate has a highly variable relationship to the level of physical activity, and this relationship can also vary within and between individuals.

Thus, heart-rate can be a good general indicator of activity, but can not be a very precise indicator of energy expenditure. Moreover, analysing heart-rate records, even using computerized techniques, often requires some expert decision-making and is a tedious procedure. An additional problem with young infants is that they tend to tamper with electrodes and connecting electrical wiring. We had to discontinue a study on 2-3-year-olds for this reason. Heart-rate recording has a definite place in the methodology of assessing physical activity in children, but has to be employed with discretion.